Normally the two whale species—both of which can reach 35 feet (11 meters) in length—undertake seasonal migrations that separate them by many miles of ocean.

Northern minkes head toward the North Pole in spring and ply waters up to the edge of Arctic ice during the summer. In autumn these whales head south, nearly as far as the Equator, to spend the winter. (See whale pictures.)

Antarctic whales follow a similar pattern, moving between Antarctic ice and warmer mid-latitudes with the seasons.

But because the two hemispheres' seasons are opposite, the minke species don't share near-equatorial waters at the same time. Thus, they were never thought to meet—until now.

Whale of a Shocker

Minkes have been hunted extensively since the 1930s, and the few nations that still practice whaling—including Norway, Greenland, and Japan—target them today.

Soon after Norwegians resumed commercial whale hunting in 1993—following a brief moratorium—the country established a DNA registry to analyze whale kills and help ensure that whale products come from legal sources.

Geneticist Kevin Glover was recently analyzing whale DNA when he came across a surprise—a whale hunted in the northeastern Atlantic in 2007 had the genetic blueprint of a hybrid, with an Antarctic minke mother.

Glover's colleague then told him an interesting story relayed by a whaling vessel's scientific observer nearly 15 years ago.

"He said there was a very strange-looking individual taken back in 1996—it didn't have the white patch on its pectoral flippers like the [northern minke whales do]," said Glover, of the Institute of Marine Research in Bergen, Norway. "I wonder if it could be the same sort."

So Glover analyzed the DNA of the 1996 whale captured in the North Atlantic, and found a shocker: It was a pure Antarctic whale. The sample had been overlooked because the DNA archive was in its infancy when the whale was captured.

This Antarctic whale in the Arctic provided further evidence that Antarctic minkes can migrate to the home waters of their northern relatives and—as the hybrid shows—even mate with them, according to the study, published December 22 in the journal PLoS ONE.

Drop in Food Driving Whale Migration?

Still, the genes leave a lot of unanswered questions—Is the hybrid whale a fluke, or the beginning of a trend? No one knows, but Glover said that his whale biologist colleague Nils Øien has an interesting theory.

Japanese studies showed that the numbers of Antarctic minke in the Southern Hemisphere appeared to drop significantly between the 1980s and 1990s. Other studies show that supplies of the krill—tiny marine crustaceans—that fuel the Antarctic food chain also dropped during this period.